The invention relates to a microwaveable plastic wrap having an adhesive or cling layer, and a method for making the plastic wrap. The plastic wrap has a good balance of cling properties with handling properties such as tear-bar resistance and the ability to separate the plastic wrap from itself. The cling layer exhibits sufficient cling force to stick to different substrates such as, for example, glass, plastic, ceramic, stainless steel, laminated cardboard, and aluminum.
Thermoplastic resin films are used as household plastic wraps to contain food and other items by clinging to one or more surfaces of the container in which the food is housed or to itself to thereby provide a seal which reduces the degree of exposure of the contents of the container to the environment. A necessary characteristic of such plastic wrap film is the ability to xe2x80x9ccling,xe2x80x9d that is, to adhere to itself or to form a tight seal with other surfaces, as when the film is used as a cover for containers. Plastic wrap films can be employed to preserve the freshness and moisture content of wrapped food items and to prevent the migration of food odors.
Thermoplastic resin films do not generally possess inherent cling characteristics sufficient to adhere satisfactorily to themselves and other surfaces. The cling characteristic of thermoplastic films is generally obtained through the use of so-called cling agents or adhesives. Adhesives are chosen for their ability to produce a surface on a thermoplastic film that can be sealed, opened and resealed, and are selected with due care in consideration of the use of thermoplastic films in direct food contact applications.
A balance must be attained between the film""s xe2x80x9cclingxe2x80x9d and xe2x80x9chandlingxe2x80x9d or xe2x80x9chandleabilityxe2x80x9d characteristics. xe2x80x9cHandleabilityxe2x80x9d is the characteristic of the film to resist inadvertent clinging to itself and to other surfaces during use. While adhesives impart good cling properties to the film, it has generally been observed that an increase in the amount of adhesives incorporated into the film to enhance the cling provides a corresponding decrease in the handleability of the film. With an increased adhesive amount, the tendency of the film to inadvertently adhere to itself and to other surfaces increases. One type of inadvertent sticking is known as premature sticking; that is, before the adhesive film can be properly positioned over a target surface, inadvertent contact of the adhesive with the target surface or the plastic wrap itself causes premature sticking at one or more positions, thereby inhibiting proper positioning of the plastic wrap for use.
Finally, with the increased use of microwave ovens, it has become desirable for plastic cling wraps to be used in microwave applications. All plastic wraps today are not suitable for such use. For example, standard polyethylene wraps will melt in microwave ovens after two-three minutes of microwave exposure.
It is an object of the invention to provide a multi-layer plastic wrap having a cling layer that exhibits sufficient cling to different substrates such as, for example, glass, plastic, ceramic, stainless steel, laminated cardboard, and aluminum.
It is a further object of the invention to provide a plastic wrap which exhibits a good balance of cling properties with handling properties such as tear-bar resistance, and the ability to separate the plastic wrap from itself.
It is a further object of the invention to provide a plastic wrap which includes a cling layer integral to the film which provides an adhesive that remains with the film upon removal from the container to which it had been sealed.
It is yet another object of the invention to provide a microwaveable plastic wrap that is acceptable for direct food contact.
It is another object of the invention to provide a method for producing the plastic wrap.
Other objects of the invention will become apparent to those skilled in this art from the following description.
In its broadest sense, the invention relates to a plastic wrap which includes a release layer made from a polyolefin resin. The polyolefin resin should have a sufficiently high melting point and resistance to heat for direct food contact microwave re-heating. Examples of polyolefin resins that can be used to make the release layer of the present invention plastic wrap include polypropylene homopolymer or polypropylene/ethylene copolymer. The release layer also should contain an effective amount of an anti-blocking agent to provide sufficient release properties to the release layer to facilitate dispensing of the plastic wrap from a dispensing roll.
The plastic wrap should further include a cling layer and an intermediate or core layer positioned between the release and cling layers. The cling layer should contain a base resin and an effective amount of a tackifier to provide sufficient cling to different substrates such as, for example, glass, plastic, ceramic, stainless steel, laminated cardboard, and aluminum. The base resin may be an ethylene vinyl acetate (xe2x80x9cEVAxe2x80x9d), or preferably an ethylene methyl acrylate (xe2x80x9cEMAxe2x80x9d), resin. The tackifier may be a styrene-isoprene-styrene (SIS) block copolymer in combination with a rosin ester tackifier. The core layer can be made from a polyolefin resin such as HDPE, LDPE, LLDPE, and mixtures thereof.
The intermediate layer of the plastic wrap may be bonded to the outer release layer and cling layer using any suitable bonding materials or methods now known or later developed. The plastic wrap may be coextruded, as opposed to laminated, and a multi-manifold slot cast die may be employed to form the film. Alternatively, an air blown film tubular co-extrusion method may be used. Before extrusion, the different materials comprising the various layers may be mixed. The mixed components of each of the layers may be loaded into a hopper and then gravity fed into an extruder, although the base resin and tackifier may require compounding before being fed into the extruder as well.
One embodiment of the invention may use a single-manifold slot cast die and multilayer adapter for the slot cast die. An alternative embodiment may feed a hot melt adhesive including the base resin and the tackifier in a liquid form directly through a die adapter and to the die, where it may be joined with the extruded release and intermediate layers to form a film. Commercial hot melt adhesives that include tackifiers and carrier resins are available from 3M, among other vendors, or hot melt adhesives may be custom blended to include desired tackifiers and carrier resins.
The invention also relates to a method for making the plastic wrap. The film may be produced by mixing a first polyolefin and an antiblocking agent to form a first polyolefin composition, mixing a base resin and a tackifier, feeding the first polyolefin composition into an extruder to form a first extruded layer, feeding a second polyolefin into another extruder to form a second extruded layer, feeding the base resin and tackifier mixture into yet another extruder to form a third extruded layer, and joining the first extruded layer, the second extruded layer, and the third extruded layer to form a film, preferably substantially simultaneously.
The film obtained from the extrusion may be cooled, and then embossed to form the final plastic wrap. Cooling may take place by contacting the film on a chill roll for the slot cast die methods, or air cooling for the air blown film tubular co-extrusion method. Post-extrusion embossing requires a second heating of the film on a heated steel embossing roll for thermosetting, followed by a second quench on a chill roll to set the embossment. One advantage to post-extrusion embossing is that product identification markings may be added by embossing, as opposed to printing such markings on the plastic wrap as is typically done in the industry. The act of printing can have adverse effects on the property of the plastic wrap. Alternatively, the plastic wrap may be embossed as it exits the die.
The plastic wrap is advantageous because it is microwaveable, it clings to a variety of different surfaces and it is easily dispensed from a dispensing roll.
As used in this specification, good tear-bar resistance is considered the tendency of a plastic wrap to tear as measured by techniques known in the art. The ability to separate plastic wrap from itself is measured by a plastic wrap""s tendency to cling to itself, as measured by techniques known in the art.